1. Field of the Invention
The present invention generally relates to a signal distribution method and a signal distribution apparatus, and especially relates to a signal distribution method and a signal distribution apparatus for distributing a television signal transmitted on synchronous networks, such as SONET/SDH (Synchronous Optical Network/Synchronous Digital Hierarchy).
2. Description of the Related Art
Conventionally, a television signal is distributed from a CATV (CAble TeleVision) station and a DTV (Digital TeleVision) station to users (subscribers) based on the analog principle using a 64QAM (Quadrature Amplitude Modulation) method.
FIG. 1 is a block diagram of an example of a conventional television signal distribution system, wherein a CATV head-end station 10 receives a television signal from a source such as a satellite, an IRT (Integrated Receiver Transcoder) 11 modulates the television signal by the 64QAM modulation, a converter 12 converts the modulated signal into an RF signal, a multiplexing unit 13 multiplexes the RF signal and other television signals, and a transmitting unit 14 transmits the multiplexed television signal, which is an analog RF signal, to a network 15. Further, a receiving unit 16 receives the analog RF signal from the network 15, and distributes (splits) the analog RF signal to two or more lines through a distributing unit 17, and amplifiers 18 amplify the distributed (split) RF signal for transmitting the RF signal to each user (CATV subscriber) through each coaxial cable 19.
The conventional analog method has problems in that a large number of channels cannot be served, and a cable telephone service cannot be offered. Recently and continuing, the signal distribution method from the head-end station is being gradually changed from analog to digital, using a CODEC.
FIG. 2 is a block diagram of another example of the conventional television signal distribution system, wherein the head-end station 20 receives a television signal from a source such as a satellite, and an IRT 21 modulates the television signal by the 64QAM modulation. The 64QAM signal and another 64QAM television signal from a video switch 22 are provided to a CODEC/transmitting unit 23.
The CODEC/transmitting unit 23 encodes and digitizes the 64QAM signals, multiplexes the digitized signals, carries out electric/optical conversion such that an optical signal having a transmission rate, e.g., 2.38 Gbps is generated. The optical signal is provided to an optical multiplexing unit 24 for further multiplexing, and transmitted to an optical transmission line 25.
An optical demultiplexing unit 26 demultiplexes the optical signal transmitted via the optical transmission line 25 based on wavelengths. A CODEC/receiving unit 27 carries out optical/electric conversion of the demultiplexed optical signal, and further demultiplexes to acquire each channel. Decoding is performed on each channel such that analog 64QAM signals are obtained. The 64QAM signals are converted to an RF signal by a converter 28, and the RF signal is distributed (split) to two or more lines by a distributing unit 29. Amplifiers 30 amplify the corresponding split signals for distribution, one to each user (subscriber) through corresponding coaxial cables 31.
Here, each of the 64QAM television signals is not compressed, and occupies a bandwidth for 155.52 Mbps per channel. The CODEC/transmitting unit 23 multiplexes the 64QAM television signals into a 2.388 Gbps signal, and the multiplexer 24 multiplexes N 2.388 Gbps signals into an Nx2.388 Gbps signal (N being an integer).
According to this conventional system, the bandwidth of 155 Mbps is needed per channel since the television signal is not compressed; and since the transmission rate of the conventional system is different from the transmission rate of SONET/SDH, a cable telephone service and the like are not available, affinity of the conventional system to SONET/SDH being poor.
In order to solve this problem, there is a method, wherein a CODEC apparatus and a SONET/SDH transmission apparatus are used. FIG. 3 is a block diagram of another example of the conventional television signal distribution system.
In FIG. 3, a head-end station 40 receives a television signal from a source such as a satellite, and an IRT 41 modulates the received television signal into a 64QAM signal, which is supplied to a CODEC/transmitting unit 43. Further, another 64QAM television signal is provided from a video switch 42 to the CODEC/transmitting unit 43.
The CODEC/transmitting unit 43 encodes and digitizes each of the 64QAM television signals, multiplexes the encoded and digitized signals, and carries out electric/optical conversion. Each of the encoded and digitized signals is mapped to a path STS-xx of SONET/SDH at the rate of 155 Mbps per channel, provided to a SONET/SDH transmitting unit 44, and transmitted to an optical transmission line 45 serving as a communication line OC-xx (Optical Carrier-xx) of the SONET/SDH system from the SONET/SDH transmitting unit 44.
A SONET/SDH receiving unit 46 receives the optical signal transmitted via the optical transmission line 45, and the electrical signal mapped by the path STS-xx of SONET/SDH is obtained. The electrical signal is demultiplexed by a CODEC/receiving unit 47, and decoded for every channel such that the analog 64QAM signals are obtained. The 64QAM signals are converted into an RF signal by a converter 48. The RF signal is distributed (split) to two or more lines by a distributing unit 49, and is distributed to each user.
This is a costly method as compared with the conventional analog method, and management of the CODEC apparatuses and the SONET/SDH apparatuses becomes complicated.
In an attempt to solve this problem, a method is proposed wherein MPEG-2TS (Transport Stream) packets of the MPEG (Moving Picture Exports Group) specification are transmitted via a SONET/SDH transmission network in the transmission format of DVB-ASI (Digital Video Broadcasting ASynchronous Interface), which is a European digital broadcasting format. For example, Patent Reference 1 discloses a method such as described above.
[Patent Reference 1] JP, 10-190767, A
[Description of the Invention]
[Problem(s) to be Solved by the Invention]
According to the method wherein MPEG-2TS packets are transmitted over a SONET/SDH transmission network in the transmission format of DVB-ASI, since a television channel can be compressed to a minimum size of 3.75 Mbps per channel, a maximum of 72 channels can be received by each DVB-ASI of a SONET/SDH transmission apparatus.
In order that a CATV station can provide hundreds of 3.75 Mbps MPEG-2TS packet channels to the DVB-ASI interface of the SONET/SDH transmission apparatus, the DVB-ASI interface is prepared with a sufficient number of ports. However, not all of the ports are fully loaded with 72 channels, but some ports may carry only a few channels. Here, 3.75 Mbps×72 channels=270 Mbps, which is equal to the maximum receiving data rate of the DVB-ASI.
Usually, when assigning a bandwidth for the maximum data rate 270 Mbps of DVB-ASI in the SONET/SDH network, STS-1-6V (311.04 Mbps) is assigned in SONET, and STM-1-2V (311.04 Mbps) is assigned in SDH.
Even if an actual requirement for the bandwidth is less than 311.04 Mbps, a customer (CATV operator) has to purchase (or lease, and so on) an STS-1-6V. That is, the problem is that there is no method available for assigning a band of the SONET/SDH optimally and flexibly according to customer (CATV operator) requirements.
Further, even if a CATV station is able to transmit MPEG-2TS packets via a narrowed bandwidth, i.e., partially loading STS-1-6V, the SONET/SDH apparatus on the receiving side, which expects to receive data at the 311.04 Mbps data rate, has to be equipped with some compensating means for receiving the partially loaded STS-1-6V. The problem is that there is no specific processing method available.